The present invention relates to a device for forming a spiral groove on the inner wall of a cylindrical cavity, and more particularly to a device for forming a lubricating-oil-guiding spiral groove on the inner wall of a bearing's cavity which abrades the outer surface of a shaft.
Usually, lubricating oil is coated on the inner wall of a cylindrical cavity of a radial bearing through which a cylindrical shaft passes so as to reduce friction due to the sliding of the shaft against the inner wall of the cavity. The lubricating oil forms a thin oil film between the outer surface of the shaft and the inner wall of the cavity to prevent friction between them and to prevent the abrasion of portions thereof.
When using lubricating oil, the state of the oil film formation is very important to effectively protects parts which undergo friction. If parts of an oil film do not form, friction increases in those portions, to cause severe abrasion, thus causing the support between the oiled items, e.g., a shaft and bearing, to become unstable. To prevent this, a lubricating oil guiding groove for stably supplying the lubricating oil is formed on the inner wall of the bearing's cavity. The amount of lubricating oil supplied and the state of oil film formation vary according to the depth of the guiding groove. The deeper the groove is, the more lubricating oil should be supplied. However, if the groove is too deep, lubricating oil is wasted because it is supplied excessively, and a stable and uniform oil film cannot be formed. If the groove is too shallow, the lubricating oil is not wasted but the supply thereof is not stable enough to form a consistent oil film. Therefore, the groove should be formed by a precise machining device.
FIG. 1 is a conventional device for forming a lubricating-oil-guiding spiral groove on the inner wall of a radial bearing's cavity.
The groove-forming device has a rod 1 whose machining portion 1c placed at the front end thereof is put into a cylindrical cavity of a bearing 5. A cylindrical portion 1a of a predetermined diameter is formed at the end face of machining portion 1c. Cylindrical portion 1a is plugged with a threaded cap 4. The plugged cylindrical portion 1a communicates with the exterior via two through holes 1b. As machining tips, machining balls 2 are provided in through holes 1b. A rod-shaped support 3 for supporting machining balls 2 is provided in cylindrical portion 1a. Machining balls 2 protrude a predetermined height from the external surface of machining portion 1c due to support 3 so as to make pressed contact with the inner surface of the cavity of bearing 5 which is an object to be machined. Machining balls 2 have a hardness higher than that of the object to be machined, i.e., bearing 5.
In order to form a guiding groove on the inner surface of the cavity of bearing 5, the rod is rotated at a predetermined speed and the machining portion at the end thereof is put into the bearing's cavity also at a predetermined speed. The machining balls of the machining portion are spirally moved to form the spiral shape in the inner surface of the cavity. By doing this, the guiding groove is formed.
However, with the conventional device for forming a lubricating-oil-guiding groove in a bearing, bearings only of certain diameters can be machined since the protruding height of the machining balls is fixed. Thus, undesirably, bearings of widely varying diameters cannot be machined, using one device, which makes guiding-groove-forming devices of various sizes necessary.